i don;t usually suggest this but as of Co2 guns that use 12g cartridges Daisy and Crosman makes the best. but don't be fooled by this and go buy Pulse AEGS they're trash. ONLY get co2 12g cartridge using guns from Crosman or Daisy
well there are spring parts in the gun. I wish i couldve drawn you a picture.
so you install the co2 cartridge into the gun and by screwing it firmly into the gun's air nozzle you puncture the hole into the mouth of the cartridge.
then the gun's air nozzle holds the air in the cartridge until you pull the trigger, which then lets out a small amount of compressed air from the cartridge and into the barrel. of course the trigger is powered by a spring to let the air release from the cartridge. However, the power to the BB actually comes from the released gas. because gas is compressed, it shoots out of the cartridge rather fast, and with the barrel being its only release point, it rushes toward the end of the barrel, puhsing the BB along.
this is the case with most gas-driven guns, except those without cartridges the gas is stored somewhere else other than the cartridge, usually the magazine.
as of power, Co2 is pretty lethal with fresh cartridges installed. with smaller, metal .117 calibers, rather than plastic 6mms, an average gas pistol will shoot about 400-500FPS, which is pretty darn fast. this is actually more than enough to kill small games 50feet of less. however, most airsoft games don't allow metal BBs, so consider. with the 6mms, you will get no more than 400FPS, and because the projectile is nonlethal it tends to be more favorable for skirmishes.
i am confidant to say that ALL spring pistols suck, with the only exception of DE 1911 models. this only shoots about 200FPS, but the integrated (non-adjustable, though) hopup is pretty well balanced and allows the BBs range of about 100feet, pretty accurately. but it's fully plastic and parts break rather easy. you're very extremely fortunate if you get 300FPs out of a spring pistol.
enough said